The present invention is directed to techniques related to communication networks. More specifically, specific embodiments of the present invention are directed to multiple phase-shifting keying modulation. For example, various embodiments of the present invention are used for modulating data for the purpose of transmitting data over optical communication networks. In a specific embodiment, the present invention provides a scheme for quadrature phase-shift keying modulation in an optical network. But it is to be understood that techniques according to the present invention have a broad range of applications. For example, embodiments of the present invention can be used for binary phase-shift keying modulation, higher-order (i.e., eight or higher) phase-shift keying modulation, differential phase-shift keying modulation, and other types of phase-shift keying modulation (e.g., DQPSK, etc.)
Phase-shift keying (PSK) modulation is a widely adopted modulation technique used in various types of communication networks. In particular, PSK modulation method is particularly useful for optical networks. Among other things, PSK modulation allows a signal bandwidth to be narrow, thereby increases transmission efficiency in a cost-effective manner. In addition, PSK modulation is suitable for optical networks because to a large degree it tolerates nonlinearity (e.g., caused by physical properties of optical media) in optical communication networks.
In a PSK modulation scheme, a finite number of phases are used. A unique pattern of binary bit(s) is assigned to each of the phase. For example, in a binary phase-shift keying (BPSK) modulation scheme, two phases separated by π are used, and these two represent “0” and “1” respectively. As another example, in a quadrature phase-shift keying (QPSK) modulation scheme, four phases separated by π/2 are used, and these four phases respectively represent “00”, “01”, “10”, and “11”. In higher-order phase-shift keying modulation schemes, more phases separated by smaller interval are used to represent more patterns.
It is often desirable to have PSK modulation scheme to have a large number of phases, as larger number of phases usually mean that each individual phase can be used to represent more bits. For example, in a BPSK modulation scheme, each phase represents one bit. In contrast, in a QPSK modulation scheme, each phase represents two bits, and so on and so forth. Other things being equal, by using QPSK instead of BPSK, the rate of data transmission can be greatly improved, which can be as much as being doubled.
Over the past, various conventional techniques have been developed to implement QPSK modulation schemes in optical networks. For example, Alcatel-Lucent in the United States developed an optical QPSK system. FIG. 1 is a simplified diagram illustrating a conventional optical QPSK system.
As shown in FIG. 1, a conventional optical QPSK system 100 as developed by Alcatel-Lucent includes the following components:
1. a light source 101;
2. signal sources 102, 104, and 106;
3. a phase-shift modulator 103;
4. voltage sources 105 and 107;
5. a Mach-Zehnder modulator 108; and
6. an output 109.
During operation, the light source 101 generates a light signal (e.g., a laser light signal) and sends the signal to the modulator 103. The modulator 103 modulates the received light signal using data signals generated by the signal source 102. After modulation by the modulator 103, the light signal contains information that is represented by two possible phases: 0 and π/2. The modulator 103 sends the modulated light signal to the Mach-Zehnder modulator (MZM) 108. Usually, for the MZM 108 to work for this conventional system, the MZM is a dual arm MZM apparatus. In addition to the modulated light signal, the MZM 108 also receives four additional inputs: two data signals that are compliment of each other and two voltages. Using the two complimentary data signals, the MZM 108 modulates the already modulated light signal for the second time. Typically, the two voltages are biased at a low voltage level that is associated with the modulator 108, thereby allowing the modulator 108 to modulate the already modulated data signal. As an example, FIG. 2 is a collection of graphs illustrating the operation of the system 100.
The conventional system 100 is operational in various aspects, and it is only one of many conventional QPSK systems that is available. For example, Japan's Institute for Information and Communication has provided another QPSK system for providing modulation in an optical communication network. FIG. 3 is a simplified diagram illustrating a conventional QPSK modulation device for data transmission over optical networks. For example, the conventional QPSK modulation system 300 is a QPSK system developed by the Japan's Institute for Information and Communication.
As shown in FIG. 3, the system 300 includes the following components:
1. a non-return to zero (NRZ) signal generator 301;
2. amplifiers 302, 303, 306, 307, 308, and 309;
3. splitters 304 and 305;
4. delay components 310;
5. voltage sources 312 and 313;
6. modulator 315;
7. light source 311; and
8. output 316.
The generator 301 provides two NRZ data signals that are complimentary to each other. The two NRZ data signals are then amplified by amplifiers 302 and 303 respectively. Usually, the amplifiers 302 and 303 are independent and have different gain factors. The amplified data signals are then split into two set of signals by splitters 304 and 305 respectively. Each set of signals are separately amplified again by individual amplifiers. One of the signals from each set of signals is then delayed by the delaying component. Four signals, two delayed and two undelayed signals, are then sent to the modulator 312. In addition to the received signals, the modulator receives bias voltage inputs. Using the four signals and bias voltage, the modulator 312 generates a QPSK modulated signal.
Depending upon application, modulator 312 may be implemented in various ways. For example, FIG. 4 is a simplified diagram illustrating a conventional combinational MZM modulator. As an example, the modulator 400 is used in the system 300. With four signal inputs, the modulator 400 uses two signals for phase modulation at half phase level, and uses the other two signals for phase modulation at quarter phase level. In comparison to the system 100, the system 300 has various advantages. Among other things, the system 300 provides a multitude of possibilities for external controls.
While conventional systems and techniques, such as ones explained above, are usable for various applications, they are often inadequate for various reasons, which are detailed below.
Therefore, it is desirable to have an improved system and method for m-ary PSK modulation.